Method and system using a conference bridge for handoff of a multi-mode mobile station

ABSTRACT

A multi-mode mobile station includes a first interface for wireless communication with a wireless local area network (WLAN) and a second interface for wireless communication with a wireless wide area network (WWAN). The multi-mode mobile station is associated with a particular conference bridge, through which all calls to and from the multi-mode mobile station are routed. For example, when a call involving the multi-mode mobile is established via the WLAN, the call includes a WLAN call leg that extends from the conference bridge to the multi-mode mobile station via the WLAN. To effect a handoff during the call, a WWAN call leg is established between the multi-mode mobile station and the conference bridge via the WWAN. The conference bridge bridges the WLAN and WWAN call legs. To complete the handoff, the multi-mode mobile station drops the WLAN call leg and continues the call using the WWAN call leg.

BACKGROUND

1. Field of the Invention

The present invention relates to telecommunications and, moreparticularly, to a method and system that uses a conference bridge forhandoff of a multi-mode mobile station from a first wireless network toa second wireless network.

2. Description of Related Art

There is an increased interest in using more diverse types of wirelessaccess technologies for mobile devices. Different wireless accesstechnologies may be used to provide wireless coverage in differentgeographic areas. In addition, different wireless access technologiesmay be used to provide different capabilities for transmitting andreceiving voice, data, and/or other media. For example, wireless widearea networks (WWANs), which often use wireless access technologies suchas CDMA, TDMA, or GSM, typically provide wireless coverage in relativelylarge geographic areas. However, in many cases, WWANs do not providegood wireless coverage in buildings. In addition, many WWANs have arelatively limited bandwidth for transmitting and receiving media.However, wireless local area networks (WLANs), which may use wirelessaccess technologies, such as IEEE 802.11, Bluetooth, HiperLAN, orHomeRF, have been used to provide wireless coverage in more limitedareas, such as the inside of buildings. In addition, WLANs can oftentransmit and receive media at a higher rate than many WWANs.

With this increased interest in using diverse wireless accesstechnologies to provide wireless coverage has come an increased interestin providing multi-mode mobile stations that can communicate using morethan one type of wireless access technology. For example, a multi-modemobile station may have one interface for communication with a WWAN,using a wireless access technology such as CDMA, and another interfacefor communication with a WLAN, using a wireless access technology suchas IEEE 802.11. Although such multi-mode mobile stations can providebetter wireless coverage in more areas by being able to communicate overdifferent wireless networks, they do not necessarily change theirnetwork connectivity in a seamless manner. For example, while engaged ina call via a first wireless network, the multi-mode mobile station maymove into an area in which the first wireless network no longer providesgood wireless coverage but the second wireless network does. In thatsituation, it would be desirable for the multi-mode mobile station to beable to continue the call via the second wireless network.

Accordingly, there is a need for methods and systems that can facilitatehandoffs of multi-mode mobile stations, for example, from a WWAN to aWLAN or vice versa.

SUMMARY

In a first principal aspect, an exemplary embodiment of the presentinvention provides a method for effecting a handoff of a multi-modemobile station from a first wireless network to a second wirelessnetwork. The multi-mode mobile station has a first interface forwireless communication with the first wireless network and a secondinterface for wireless communication with the second wireless network.In accordance with the method, a call request is received that requestsestablishment of a call between the multi-mode mobile station and anendpoint. In response to the call request, a first call leg isestablished between a conference bridge and the multi-mode mobilestation via the first wireless network. A handoff request is received.In response to the handoff request, a second call leg is establishedbetween the conference bridge and the multi-mode mobile station via thesecond wireless network. The conference bridge bridges the first andsecond call legs. The first call leg is dropped.

In a second principal aspect, an exemplary embodiment of the presentinvention provides a method for communicating with multi-mode mobilestations that can communicate with at least a first wireless network anda second wireless network. In accordance with the method, each one of aplurality of the multi-mode mobile station is associated with aparticular one of a plurality of conference bridges. A call request isreceived that requests establishment of a call between an endpoint and agiven one of the multi-mode mobile stations. In response to the callrequest, a given conference bridge that is associated with the givenmulti-mode mobile station is identified from among the plurality ofconference bridges. The call is established through the given conferencebridge.

In a third principal aspect, an exemplary embodiment of the presentinvention provides a system for providing wireless telecommunicationsfor a plurality of multi-mode mobile stations. The system comprises afirst wireless network, a second wireless network, a plurality ofconference bridges communicatively coupled to the first and secondwireless networks, a database, and a call control system that iscommunicatively coupled to the database and to the plurality ofconference bridges. The database associates each one of the plurality ofmulti-mode mobile stations with a particular one of the plurality ofconference bridges. The call control system selectively routes calls toand from the multi-mode mobile stations through their associatedconference bridges.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a wireless telecommunicationssystem, in accordance with an exemplary embodiment of the presentinvention;

FIG. 2 is a flow chart illustrating a method for establishing a callbetween an endpoint and a multi-mode mobile station via a WWAN, inaccordance with an exemplary embodiment of the present invention;

FIG. 3 is a flow chart illustrating a method for handing off the callestablished in FIG. 2 from the WWAN to a WLAN, in accordance with anexemplary embodiment of the present invention;

FIG. 4 is a flow chart illustrating a method for establishing a callbetween a multi-mode mobile station and an endpoint via a WLAN, inaccordance with an exemplary embodiment of the present invention;

FIG. 5 is a flow chart illustrating a method for handing off the callestablished in FIG. 4 from the WLAN to a WWAN, in accordance with anexemplary embodiment of the present invention; and

FIG. 6 is a flow chart illustrating a method for re-establishing adropped call leg, in accordance with an exemplary embodiment of thepresent invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 1. Overview

The present invention, in its preferred embodiments, uses conferencebridges to facilitate handoffs of multi-mode mobile stations between afirst wireless network and a second wireless network. The first andsecond wireless networks use different wireless communicationtechnologies. For example, the first wireless network could be awireless local area network (WLAN) that uses a protocol such as IEEE802.11x, HiperLAN, HomeRF, Bluetooth for wireless communication. Thesecond wireless network could be a wireless wide area network (WWAN)that uses, for example, CDMA or GSM for wireless communication. Thus, amulti-mode mobile station may have at least a first interface forwireless communication with the first wireless network and a secondinterface for wireless communication with the second wireless network.

In an exemplary embodiment, a plurality of conference bridges arecommunicatively coupled to the first wireless network and to the secondwireless network, and each multi-mode mobile station is associated witha particular conference bridge from among the plurality of conferencebridges. The plurality of conference bridges may be provided by one ormore network elements, such as media servers or media resource functions(MRFs). The conference bridges may convey media in a packet format. Forexample, the conference bridges could be provided by a media server thatis coupled to the first wireless network via a packet-switched networkand to the second wireless network via the packet-switched network, amedia gateway, and a circuit-switched network. The media gateway mayconvert between the media formats used in the packet-switched andcircuit-switched networks.

The multi-mode mobile stations may be associated with particularconference bridges through the use of conference bridge identifiers. Forexample, each conference bridge may be given a conference bridgeidentifier, so as to define a plurality of conference bridgeidentifiers, and each multi-mode mobile station may be assigned (eitherstatically or dynamically) one of these conference bridge identifiers. Adatabase may store information regarding which multi-mode mobilestations are associated with which conference bridges. For example, thedatabase may store a mobile station identifier for each multi-modemobile station, e.g., a mobile directory number (MDN), mobile stationidentification (MSID), and/or electronic serial number (ESN), such thatthe mobile station identifier is mapped to the conference bridgeidentifier that has been assigned to that multi-mode mobile station.

A call control system may facilitate the routing of calls to or frommulti-mode mobile stations through their associated conference bridges.For example, when a request is made to establish a call between anendpoint and a multi-mode mobile station, whether originated by theendpoint or by the multi-mode mobile station, the call control systemmay query the database to determine what conference bridge identifier isassigned to that multi-mode mobile station. The call control system maythen route that call through the conference bridge associated with themulti-mode mobile station. In this way, when the call is establishedbetween the multi-mode mobile station and the endpoint, the voice orother media for the call is conveyed via the conference bridge. Thus, ifthe call is established via the first wireless network, the call isestablished with a first call leg that extends from the conferencebridge to the multi-mode mobile station via the first wireless network.

At some point during the call, the multi-mode mobile station may requesta handoff. This may occur, for example, when the multi-mode mobile isengaged in a call via the first wireless network but determines that thesecond wireless network is available and preferred. For example, themulti-mode mobile station may be moving out of the coverage area of thefirst wireless network and into the coverage area of the second wirelessnetwork.

The multi-mode mobile station may then request a handoff by making ahandoff call to a predetermined handoff number via the second wirelessnetwork. The handoff number is such that the call control systemreceives the signaling to establish the handoff call. In response tothis signaling, the call control system queries the database todetermine what conference bridge identifier is assigned to themulti-mode mobile station. The call control system then uses theconference bridge identifier to route the handoff call to the conferencebridge associated with the multi-mode mobile station. In this way, asecond call leg is established between the conference bridge and themulti-mode mobile station via the second wireless network. Moreover, theconference bridge bridges the first and second call legs so that voiceor other media from the endpoint is conveyed to the multi-mode mobilestation over both call legs, and the multi-mode mobile station can sendvoice or other media to the endpoint via either call leg.

With the first and second call legs bridged in this way, the multi-modemobile station can use either the first or the second call leg to sendand/or receive voice or other media. However, once the multi-mode mobilestation determines that the second call leg is fully operational (e.g.,by receiving media via its second interface), the multi-mode mobilestation may drop the first call leg. In this way, a handoff from thefirst wireless network to the second wireless network is effected. Inaddition, because the second call leg is established before the firstcall leg is dropped, this approach can beneficially effect a “soft”handoff between the first and second wireless networks.

In addition to facilitating handoffs, the conference bridges can be usedto facilitate the process of re-establishing unintentionally droppedcalls. For example, a call between a multi-mode mobile station and anendpoint may be established via an endpoint as described above. At somepoint during the call, the multi-mode mobile station's call leg may beunintentionally dropped, because of a temporary degradation in signalquality or for some other reason. The multi-mode mobile station detectsthe dropped call leg and, in response, calls a predeterminedre-establishment number. The reestablishment number could be the samedirectory number as the handoff number, or it could be a differentdirectory number. The call control system receives the signaling for there-establishment call, identifies the multi-mode station's conferencebridge, and routes the re-establishment call to the identifiedconference bridge. In this way, the multi-mode mobile station's call legcan be re-established. In addition, an announcement may be played to theendpoint, in order to provide notification that the multi-mode mobilestation is attempting to re-connect and/or to fill up the time requiredto re-establish the dropped call leg.

2. Exemplary Network Architecture

FIG. 1 is a simplified block diagram of an exemplary wirelesstelecommunications system 10. In FIG. 1, connections that carryprimarily voice or other media are shown as solid lines and connectionsthat carry primarily signaling are shown as dashed lines.

Wireless telecommunications system 10 includes network elements thatfunction together as a wireless local area network (WLAN) 12 and networkelements that function together as a wireless local area network (WWAN)14. WLAN 12 may provide wireless coverage in a relatively limited area,such as in a building or part of a building. In addition, WLAN 12 mayuse one or more unlicensed frequency bands, such as the unlicensedfrequency band in the 2.4 GHz range. For example, WLAN 12 may use IEEE802.11a, IEEE 802.11b, IEEE 802.11e, IEEE 802.11g, IEEE 802.11h, or IEEE802.11n (wherein these and other members of the IEEE 802.11 family ofspecifications are referred to generically herein as “802.11x”), orvariations thereof, for wireless communication. These 802.11x standardsare incorporated herein by reference. Alternatively or additionally,WLAN 12 may use IEEE 802.16, Bluetooth, HomeRF, HiperLAN, a MultichannelMultipoint Distribution Service (MMDS), or some other wireless protocolor format for wireless communication.

WWAN 14 may provide wireless coverage in a relatively large geographicarea, such as an entire city, often by using a plurality of contiguouswireless coverage areas, such as cells or sectors. The wirelesscommunication in WWAN 14 may occur in an analog format, such as theAdvanced Mobile Phone Service (AMPS), or in a digital format, such ascode division multiple access (CDMA), time division multiple access(TDMA), or Global System for Mobile communication (GSM), and it mayoccur in licensed frequency bands, such as the 1.9 GHz PCS bands.

A multi-mode mobile station 16 has a first interface for wirelesscommunication with WLAN 12 and a second interface for wirelesscommunication with WWAN 14. Thus, multi-mode mobile station 16 is ableto communicate with WLAN 12 when operating in an area served by WLAN 12and is able to communicate with WWAN 14 when operating in an area servedby WWAN 14. In some areas, the wireless coverage of WLAN 12 and WWAN 14may be overlapping, and multi-mode mobile station 16 may use anarbitration method to determine whether to communicate with WLAN 12 orwith WWAN 14.

Multi-mode mobile station 16 may be a wireless telephone,wirelessly-equipped personal digital assistant (PDA),wirelessly-equipped laptop computer, or other type of wirelesscommunication device. The first and second interfaces of multi-modemobile station 16 may each include an antenna, a radio frequency (RF)transceiver, and a communication module appropriate for communicatingusing the particular wireless technology. A communication module may beimplemented by hardware, firmware, and/or software.

WLAN 12 includes at least one wireless access point 18. Wireless accesspoint 18 provides a wireless coverage area within which wireless accesspoint 18 is able to communicate with wireless communication devices,such as multi-mode mobile station 16, over an air interface 20. Wirelessaccess point 18 may be communicatively coupled to other networkelements, e.g., via a local area network (LAN) 22. LAN 22 may carryvoice, data, and/or other media in a packet-based format. Thus, LAN 22may be connected to other communication devices that exchange voice in apacket-based format. For example, LAN 22 may be connected to avoice-over-packet (VoP) telephone 24, a personal computer 26 equippedfor audio communication, e.g., equipped with a microphone and speaker,and/or one or more other wireless access points.

LAN 22 may also be connected to a VoP system 28 that controls VoPcommunications in WLAN 12. VoP system 28 may, for example, function as aprivate branch exchange, such as an “IP-PBX,” and/or a media terminaladapter (MTA). VoP system 28 may, in turn, be communicatively coupled toa variety of wireline and/or wireless communication devices. Forexample, VoP system 28 may be connected to analog telephony devices,such as analog telephone 30, facsimile machines, and/or modems. VoPsystem 28 may also be connected to digital telephony devices, such asdigital telephone 32.

LAN 22 may be communicatively coupled to a wide area packet-switchednetwork 34, via a network access device 36. Network access device 36could be, for example, a cable modem, DSL modem, or router.Packet-switched network 34 may route packets using a network protocol,such as the Internet Protocol (IP), in combination with the UserDatagram Protocol (UDP) or Transmission Control Protocol (TCP). The IPpackets may be carried over lower level protocols, such as asynchronoustransfer mode (ATM) protocols. Protocols, such as the Real-TimeTransport Protocol (RTP), may be used to carry voice or other mediathrough packet-switched network 34 in a real-time format. Relevantaspects of RTP are described in Schulzrinne, et al., “RTP: A TransportProtocol for Real-Time Applications,” Request for Comments 1889 (January1996), which is incorporated herein by reference.

Other protocols, such as the Session Initiation Protocol (SIP), may beused to set up and/or manage communication sessions throughpacket-switched network 34. Voice, data, and/or other media may beexchanged in such communication sessions. Relevant aspects of SIP aredescribed in Rosenberg, et al., “SIP: Session Initiation Protocol,”Request for Comments 3261 (June 2002), which is incorporated herein byreference. SIP and/or other protocols may, in turn, use the SessionDescription Protocol (SDP) to describe the communication sessions thatare being set up or managed. Relevant aspects of SDP are described in M.Handley, et al., “SDP: Session Description Protocol,” Request forComments 2327 (April 1998), which is incorporated herein by reference.

In an exemplary embodiment, SIP is used to set up communication sessionsthrough packet-switched network 34 that involve WLAN 12. WLAN 12 mayinclude one or more SIP user agents for this SIP signaling. For example,VoP system 28 may include a SIP user agent to engage in SIP signaling onbehalf of multi-mode mobile station 16 or other communication devicescommunicatively coupled to WLAN 12. Alternatively, multi-mode mobilestation 16 may have its own SIP user agent.

WWAN 14 may include a base transceiver station (BTS) 38 that provides awireless coverage area within which BTS 38 may communicate with one ormore mobile stations, such as multi-mode mobile station 16, over an airinterface 40. Although FIG. 1 shows only one BTS, it is to be understoodthat WWAN 14 may include a plurality of BTSs that may provide aplurality of wireless coverage areas. The communications between BTS 38and multi-mode mobile station 16 may occur in a digital format, such asCDMA, TDMA, GSM, or they may occur in an analog format, such as AMPS.The communications could be voice only, data only (e.g., using EV-DO),or may include a combination of voice and data (e.g., using EV-DV). Apreferred wireless communications format is cdma2000 such as describedin EIA/TIA/IS-2000 Series, Rev. A (published March 2000), which isincorporated herein by reference.

BTS 38 may be controlled by a base station controller (BSC) 42, which,in turn, may be controlled by a mobile switching center (MSC) 44.Although FIG. 1 shows only one MSC and only one BSC, it is to beunderstood that WWAN 14 may include a plurality of MSCs, which may, inturn, control a plurality of BTSs, via a plurality of BSCs. MSC 44 alsohas access to a visitor location register (VLR) 46. VLR 46 stores datarecords for mobile stations, such as multi-mode mobile station 16, thatare being served by MSC 44. A data record stored in VLR 46 for a mobilestation may identify the mobile station, e.g., by mobile directorynumber (MDN), mobile station identification (MSID), and/or electronicserial number (ESN). The data record may also include status informationfor the mobile station, such as whether the mobile station is busy, andmay also include a service profile that identifies the services to whichthe mobile station subscribes. The data record may also include otherinformation relating to the mobile station. Although FIG. 1 shows VLR 46as a network element separate from MSC 44, VLR 46 may be integrated orco-located with MSC 44.

WWAN 14 may also include a home location register (HLR) 48 that stores adata record for multi-mode mobile station 16. The data record stored inHLR 48 for multi-mode mobile station 16 may identify multi-mode mobilestation 16, such as by MDN, MSID, and/or ESN and may include a lastknown location of multi-mode mobile station 16. For example, the datarecord may identify the VLR that most recently registered multi-modemobile station 16 with HLR 48. The data record may also include statusinformation for multi-mode mobile station 16, a service profile formulti-mode mobile station 16, and other information relating tomulti-mode mobile station 16.

MSC 44 is connected to the public switched telephone network (PSTN) 50.PSTN 50 may use an out-of-band signaling system, such as SignalingSystem 7 (SS7) to route calls. Thus, PSTN 50 may include acircuit-switched network 52 that carries bearer traffic, i.e., the voiceor other media in calls, and a signaling network 54 that carriessignaling traffic used to set up, tear down, monitor, and control calls.Circuit-switched network 52 may include a plurality of trunks, with eachtrunk carrying media in a time division multiplex (TDM) format.Signaling system 54 may include a plurality of networked signal transferpoints (STPs).

PSTN 50 may also be connected to various landline telephony endpoints,exemplified in FIG. 1 by landline telephone 56. More particularly,landline telephone 56 may be connected to a switching system, such asservice switching point (SSP) 58, which, in turn, may have a bearerconnection to circuit-switched network 52 and a signaling connection tosignaling network 54.

MSC 44 may communicate with signaling network 54, e.g., using SS7, toroute calls via circuit-switched network 52 to and from mobile stationsbeing served by WWAN 14, such as multi-mode mobile station 16. Toprovide telecommunications services to mobile stations being served byWWAN 14, such as multi-mode mobile station 16, MSC 44 may alsocommunicate with HLR 48 via signaling network 54. The communicationsbetween MSC 44 and HLR 48 may conform to IS-41 specifications. A recentrevision of the IS-41 specifications, ANSI/TIA/EIA-41-D-97, published inDecember 1997, is incorporated herein by reference. The IS-41 signalingmay be carried in signaling network 54 as an SS7 application layer.

Packet-switched network 34 may be communicatively coupled tocircuit-switched network 52, via a media gateway 60. Media gateway 60may convert between media formats used in circuit-switched network 52and packet-switched network 34. For example, media gateway 60 mayreceive media from circuit-switched network 52 in a TDM format andconvert the media into an RTP format for transmission overpacket-switched network 34, and vice-versa. Media gateway 60 may also beconnected to, or integrated with, MSCs, such as MSC 44. Thus, MSC 44 mayhave a bearer connection with media gateway 60 via an intermachine trunk62.

A media serer 64 may also be communicatively coupled to packet-switchednetwork 34. Media server 64 provides a plurality of conference bridges,such as conference bridges 66, 68, and 70, each of which may be able tobridge three or more call legs that extend through packet-switchednetwork 34 as three or more media streams (e.g., in an RTP format). Forexample, with multi-mode mobile station 16 involved in a call with anendpoint, conference bridge 66 might bridge together: (i) a first callleg that extends from media server 64 to multi-mode mobile station 16via packet-switched network 34, WLAN 12, and air interface 20; (ii) asecond call leg that extends from media server 64 to multi-mode mobilestation 16 via packet-switched network 34, media gateway 60,circuit-switched network 52, WWAN 14, and air interface 40; and (iii) athird call leg that extends from the endpoint, e.g., landline telephone56, to media server 64 via SSP 58, circuit-switched network 52, mediagateway 60, and packet-switched network 34.

Although FIG. 1 shows three conferences bridges in media server 64, itis to be understood that media server 64 could include a greater orfewer number of conference bridges. In addition, conference bridges 66,68, and 70 could be provided by one or more other network elements. Forexample, conference bridges 66, 68, and 70 could be provided by a mediaresource function (MRF) or by a media gateway.

Each conference bridge in media server 64 may be associated with aparticular multi-mode mobile station. More particularly, each conferencebridge may be given a conference bridge identifier that is, in turn,assigned to a particular multi-mode mobile station. For example,conference bridge 66 might be associated with multi-mode mobile station16. As described in more detail, this association may result in havingall calls to or from multi-mode mobile 16 station routed throughconference bridge 66.

Wireless telecommunications system 10 may include a conference bridgeidentifier (CBI) database 72 that keeps track of which conferencebridges are assigned to which multi-mode mobile stations. For example,CBI database 72 may store conference bridge identifiers of theconference bridges provided by media server 64 and may store mobilestation identifiers of multi-mode mobile stations so that the mobilestation identifiers are mapped to the conference bridge identifiers oftheir associated conference bridges. Such mobile station identifiers mayinclude, for example, MDN, MSID, and/or ESN. In this way, a mobilestation identifier of a multi-mode mobile station may be used to queryCBI database 72 and obtain the conference bridge identifier of theconference bridge associated with that multi-mode mobile station.

CBI database 72 may be communicatively coupled to packet-switchednetwork 34, as shown in FIG. 1. Alternatively, CBI database 72 could beprovided in telecommunications system 10 in other ways. For example, CBIdatabase 72 could be integrated into a network element such as a homesubscriber server (HSS).

Media server 64 could be controlled by a call control system 74 thatfunctions to route calls to and from multi-mode mobile stations throughtheir associated conference bridges in media server 64, for example, viapacket-switched network 34, PSTN 50, and media gateway 60. Call controlsystem 74 may include a number of functional components, such as a mediacontroller 76, a SIP server 78, a signaling gateway 80, and an IP VLR82. These components may be provided in a single network element, suchas softswitch or a call session control function (CSCF). Alternatively,these components may be distributed among multiple network elements.

Media controller 76 may function to control media gateway 60, forexample, to extend calls from PSTN 50 to packet-switched network 34, orvice versa, and may function to control media server 64 to set upspecific conference bridges for calls to and from their associatedmulti-mode mobile stations. Media controller 76 may communicate withmedia gateway 60 and media server 64 via signaling links, e.g., usingthe Media Gateway Control Protocol (MGCP), H.248/Megaco, SIP, VoiceXML,and/or other protocols.

SIP server 78 may communicate with SIP user agents (which may beincluded in multi-mode mobile stations or in other network elements,such as VoP system 28, that act on behalf of multi-mode mobile station)to set up and control voice calls and other communication sessionsthrough packet-switched network 34. For example, SIP server 78 mayfunction as a SIP registrar that registers SIP user agents through theuse of the SIP REGISTER method. SIP server 78 may also function as a SIPproxy server, e.g., to set up communication sessions using the SIPINVITE method.

Signaling gateway 80 functions to convert between the signaling formatused in packet-switched network 34, e.g., SIP, and the signaling formatused in signaling network 54, e.g., SS7 and IS-41. Thus, using signalinggateway 80, call control system 74 can route calls that originate fromPSTN 50 through packet-switched network 34 and can route calls thatoriginate from packet-switched network 34 through PSTN 50.

IP VLR 82 serves as a visitor location register for packet-switchednetwork 34, storing a data record for each multi-mode mobile stationthat has registered via packet-switched network 34. More particularly,when a multi-mode mobile station associates with WLAN 12, the multi-modemobile station may register for services via packet-switched network 34,e.g., by sending a SIP REGISTER message to SIP server 78. In response,call control system 74 may obtain a data record for that multi-modemobile station from HLR 48, e.g., by having signaling gateway 80 send anIS-41 REGNOT message to HLR 48, and then store that data record in IPVLR 82. In this way, IP VLR 82 keeps track of which multi-mobilestations are currently being served by WLAN 12.

3. Exemplary Operation

FIGS. 2-6 are flow charts illustrating exemplary methods of operation.More particularly, FIG. 2 illustrates an exemplary method ofestablishing a call from an endpoint to a multi-mode mobile station viaa WWAN, using the conference bridge associated with the multi-modemobile station. FIG. 3 illustrates an exemplary method of effecting ahandoff of the WWAN call established as in FIG. 2 so that the callcontinues over a WLAN. FIG. 4 illustrates an exemplary method ofestablishing a call originated by the multi-mode mobile station via aWLAN, using the conference bridge associated with the multi-mode mobilestation. FIG. 5 illustrates an exemplary method of effecting a handoffof the WLAN call established as in FIG. 4 so that the call continuesover the WWAN. FIG. 6 illustrates an exemplary method of using theconference bridge to re-connect the multi-mode mobile station when itscall leg has been unintentionally dropped. The examples of FIGS. 2-6assume the network architecture of FIG. 1. However, it is to beunderstood that other network architectures could be used.

With reference to FIG. 2, an exemplary process may begin when a callerdials a directory number associated with a multi-mode mobile station(e.g., its MDN), as indicated by block 100. For purposes ofillustration, it will be assumed that the caller is calling from alandline station, e.g., landline telephone 56 in FIG. 1. However, it isto be understood that the caller could also be calling from a mobilestation (via either a WWAN or a WLAN), from a wireline station coupledto a packet-switched network, or from some other endpoint.

A call control system then receives a request to establish the call tothe multi-mode mobile station, as indicated by block 102. The requestmay take the form of call set-up signaling, such as SS7 signaling forcalls from PSTN 50 or SIP signaling for calls from packet-switchednetwork 34. Thus, if landline telephone 56 is originating the call, SSP58 may generate an SS7 IAM message that identifies the multi-mode mobilestation, e.g., by its MDN, as the called party. Signaling gateway 80 incall control system 74 may then receive the SS7 IAM message viasignaling network 54.

In response to this request, the call control system queries a CBIdatabase to identify which conference bridge is associated with thecalled multi-mode mobile station, as indicated by block 104. Forexample, call control system 74 may send a query to CBI database 72 thatidentifies the called multi-mode mobile station, e.g., by its MDN. CBIdatabase 72 may then perform a look-up and respond with a conferencebridge identifier that identifies the conference bridge that has beenassigned to the multi-mode mobile station.

The call control system then routes the call to the identifiedconference bridge, as indicated by block 106. For example, if conferencebridge 66 is to be used for the call, media controller 76 of callcontrol system 74 may signal to media server 64 to prepare conferencebridge 66 for the call. In this signaling, media controller 76 mayspecify that conference bridge 66 is to be used by providing theconference bridge identifier obtained from CBI database 72.

The call control system may also determine where the called multi-modemobile station is currently operating, as indicated by block 108. Forexample, call control system 74 may determine whether the calledmulti-mode mobile station is currently being served by WLAN 12 or byWWAN 14. To do this, signaling gateway 80 may send an IS-41 LOCREQmessage to HLR 48.

In this example, the called multi-mode mobile station is being served byWWAN 14. Thus, in response to the IS-41 LOCREQ message, HLR 48 checksits data record for the called multi-mode mobile station and therebydetermines that the called multi-mode mobile station is currently beingserved by MSC 44. HLR 48 then sends an IS-41 ROUTEREQ message to MSC 44to obtain a temporary local directory number (TLDN) and forwards thisTLDN to signaling gateway 80 in response to the LOCREQ message.

Once the called multi-mode mobile station is located, the call controlsystem extends the call from the conference bridge to the calledmulti-mode mobile station, via the WWAN, as indicated by block 110. Forexample, media controller 76 may signal to media gateway 60 and mediaserver 64 to set up a voice session through packet-switched network 34,and signaling gateway 80 may use the TLDN from the LOCREQ response toroute the call through signaling network 54 (e.g., in an SS7 IAMmessage) from media gateway 60 to MSC 44.

The WWAN receives the signaling to terminate the call to the multi-modemobile station and, in response, notifies the multi-mode mobile stationof the incoming call (i.e., pages and alerts the multi-mode mobilestation). The multi-mode mobile station may then answer the call, asindicated by block 112. In this way, a WWAN call leg is establishedbetween the conference bridge and the called multi-mode mobile station,as indicated by block 114, through which the multi-mode mobile stationand endpoint can exchange voice or other media via the WWAN. Thus,landline telephone 56 and conference bridge 66 can exchange media viaSSP 58, circuit-switched network 52, media gateway 60, andpacket-switched network 34, and conference bridge 66 and multi-modemobile station 16 can exchange media via packet-switched network 34,media gateway 60, circuit-switched network 52 (or IMT 62), WWAN 14, andair interface 40.

FIG. 3 illustrates an exemplary process for effecting a handoff from theWWAN to the WLAN. At some point during the call, the multi-mode mobilestation may determine that a different wireless network (e.g., WLAN 12)is available and preferred, as indicated by block 116. The multi-modemobile station may use any of various methods to determine when to checkfor availability of the other wireless network. In some cases, themulti-mode mobile station may periodically check for availability, or itmay do so at the instance of the user. Alternatively, a triggering eventmay cause the multi-mode mobile station to determine that it shouldcheck for availability of the second wireless network. For example, themulti-mode mobile station may use information about its location ormovement to determine when to check for availability of the secondwireless network. Examples of such approaches are described in U.S.patent application Ser. No. 10/391,158, filed Mar. 18, 2003, titled“Method for Determining Availability of a Radio Network,” in U.S. patentapplication Ser. No. 10/629,406, filed Jul. 29, 2003, titled “Method forDetermining Availability of a Radio Network,” and in U.S. patentapplication Ser. No. 10/980,727, titled “Method and System forTriggering Events in a Wireless Network,” filed Nov. 3, 2004, whichapplications are incorporated herein by reference.

Once the multi-mode mobile station determines that the other wirelessnetwork is available, the multi-mode mobile station may then determinewhether it is preferred. The determination that the other wirelessnetwork is preferred could be based on pre-set preferences and/or coulddepend on various criteria, such as the quality of the link and/or theavailability of desired services (e.g., voice service) using the otherwireless network.

In response to the determination that the WLAN is available andpreferred, the multi-mode mobile station registers with the WLAN, asindicated by block 118. For example, the multi-mode mobile station maytransmit a SIP REGISTER message to SIP server 78. The multi-mode mobilestation then calls a predetermined handoff number using its WLANinterface, as indicated by block 120. If the multi-mode mobile stationincludes a SIP user agent, the multi-mode mobile station may do this bysending SIP server 78 a SIP INVITE message with a Request-URI thatincludes the handoff number. If the multi-mode mobile station does notinclude a SIP user agent, the multi-mode mobile station may use adifferent protocol to send a message to a network element that doesinclude a SIP user agent (e.g., VoP system 28), which then sends a SIPINVITE message on behalf of the multi-mode mobile station.

The call control system recognizes from the handoff number specified inthe SIP INVITE message that the caller is a multi-mode mobile station,with an associated conference bridge, that is requesting a handoff. Thecall control system then queries the CBI database to identify whichconference bridge is associated with the multi-mode mobile station, asindicated by block 122. For example, call control system 74 may send CBIdatabase 72 a query that includes an identifier of the multi-mode mobilestation (e.g., an MDN, MSID, or ESN) obtained from the SIP INVITEmessage. CBI database 72 may then respond with a conference bridgeidentifier that identifies the multi-mode mobile station's designatedconference bridge, e.g., conference bridge 66.

The call control system then sets up a voice session between theidentified conference bridge and the multi-mode mobile station via theWLAN, as indicated by block 124. To do this, media controller 76 maycommunicate with media server 64 to prepare conference bridge 66 for thevoice session, and SIP server 78 may respond to the SIP INVITE messagewith a 200 OK message. In this way, a WLAN call leg is establishedbetween the multi-mode mobile station and its associated conferencebridge, as indicated by block 126, through which the multi-mode mobilestation and the endpoint can exchange voice and/or other media. Thus,landline telephone 56 and conference bridge 66 can exchange media viaSSP 58, circuit-switched network 52, media gateway 60, andpacket-switched network 34, and conference bridge 66 and multi-modemobile station 16 can exchange media via packet-switched network 34,WLAN 12, and air interface 20.

Moreover, the conference bridge bridges the WWAN and WLAN call legs, asindicated by block 128. In particular, the conference bridge mixes thevoice or other media for the WWAN and WLAN call legs, so that mediatransmitted by the endpoint is sent to the multi-mode mobile station viaboth the WWAN and the WLAN call legs, and media that the multi-modemobile station transmits over either call leg is sent to the endpoint.

At some point, for example, when the multi-mode mobile station startsreceiving media via its WLAN interface, the multi-mode mobile stationdrops the WWAN call leg, e.g., by transmitting a release message, asindicated by block 130. In this way, the conference bridge canbeneficially facilitate a soft handoff between the WWAN and the WLAN.

Calls originating from multi-mode mobile stations may also be routedthrough their respective conference bridges, as illustrated in FIGS. 4and 5. For purposes of illustration, the multi-mode mobile station iscurrently being served by a WLAN, e.g., WLAN 12, in the example of FIGS.4 and 5. With reference to FIG. 4, the process may begin when the userof a multi-mode mobile station dials the directory number of anendpoint, e.g., landline telephone 56, as indicated by block 200.

In response, the multi-mode mobile station transmits a request, via itsWLAN interface, to establish a call to the endpoint, as indicated byblock 202. For example, the multi-mode mobile station may transmit (oranother network element may transmit on behalf of the multi-mode mobilestation) a SIP INVITE message to SIP server 78. The SIP INVITE messagemay include a Request-URI that identifies the endpoint and may identifythe calling multi-mode mobile station, e.g., by MDN, MSID, or ESN.

A call control system receives the request and recognizes (e.g., from anidentification of the multi-mode mobile station contained in therequest) that the call should be routed through the conference bridgethat is associated with the multi-mode mobile station, as indicated byblock 204. To identify which conference bridge is associated with themulti-mode mobile station, the call control system queries a CBIdatabase, as indicated by block 206. For example, call control system 74may send CBI database 72 a query that identifies the calling multi-modemobile station, and CBI database 72 may respond with a conference bridgeidentifier that specifies what conference bridge to use for the call,e.g., conference bridge 66.

The call control system then establishes a call leg between the endpointand the identified conference bridge, as indicated by block 208. Forexample, media controller 76 may signal to media server 64 to prepareconference bridge 66 for the call, and signaling gateway 80 may transmitan SS7 IAM message through signaling network 54 to route the call to thecalled endpoint, e.g., landline telephone 56.

Once the called endpoint answers, the call leg between the calledendpoint and the conference bridge is completed. The call control systemthen accepts the multi-mode mobile station's request to establish a callto the endpoint, as indicated by block 210. For example, the callcontrol system may send a SIP 200 OK message to the multi-mode mobilestation, which may then respond with an ACK acknowledgement. In thisway, a WLAN call leg is established between the conference bridge andthe multi-mode mobile station, as indicated by block 212, through withthe multi-mode mobile station and the called endpoint can exchange voiceor other media. Thus, once the call to landline telephone 56 isestablished, landline telephone 56 and conference bridge 66 can exchangemedia via SSP 58, circuit-switched network 52, media gateway 60, andpacket-switched network 34, and conference bridge 66 and multi-modemobile station 16 can exchange media via packet-switched network 34,WLAN 12, and air interface 20.

FIG. 5 illustrates an exemplary process for effecting a handoff from theWLAN to the WWAN. At some point during the call, the multi-mode mobilestation determines that the WWAN is available and preferred, asindicated by block 214. In response, the multi-mode mobile stationregisters with the WWAN, as indicated by block 216. For example, if themulti-mode mobile station is an area served by MSC 44, then themulti-mode mobile station may transmit a registration message and MSC 44may responsively send an IS-41 REGNOT message to HLR 48. The multi-modemobile station then calls a predetermined handoff number using its WWANinterface, as indicated by block 218. The signaling for the call to thehandoff number is routed to the call control system, as indicated byblock 220. For example, MSC 44 may generate an SS7 IAM message thatidentifies the handoff number as the called number. Signaling network 54may then route the SS7 LAM message to signaling gateway 80, e.g., basedon a point code for signaling gateway 80 that is associated with thehandoff number.

The call control system receives this signaling and recognizes (e.g.,from an identification of the multi-mode mobile station) that the callshould be routed through the multi-mode mobile station's conferencebridge. To identify which conference bridge is associated with themulti-mode mobile station, the call control system queries the CBIdatabase, as indicated by block 222. The call control system then routesthe call to the identified conference bridge, as indicated by block 224.

In this way, a WWAN call leg is established between the multi-modemobile station and the conference bridge, as indicated by block 226,through which the multi-mode mobile station and the called endpoint canexchange voice or other media. Moreover, the conference bridge bridgesthe WWAN and WLAN call legs, as indicated by block 228. As a result, themulti-mode mobile station can receive media from the endpoint via eithercall leg and can transmit media to the endpoint via either call leg.

At some point, for example, when the multi-mode mobile station startsreceiving media via its WWAN interface, the multi-mode mobile stationdrops the WLAN call leg, as indicated by block 230, e.g., byde-registering with SIP server 78. To de-register, multi-mode mobilestation 16 may, for example, transmit a SIP REGISTER message with anexpiration period of 0. The multi-mode mobile station may do this, forexample, when it begins to receive media via its WWAN interface.Alternatively, the multi-mode mobile station may transmit a SIP BYEmessage.

Once the WLAN call leg is dropped, the multi-mode mobile stationcontinues using the WWAN call leg for the call. In this way, a softhandoff from the WLAN to the WWAN may be effected.

The approach of routing calls to and from a multi-mode mobile stationthrough its associated conference bridge can also facilitate the processof re-connecting the multi-mode mobile station when its call leg hasbeen unintentionally dropped, as illustrated in FIG. 6. The process maybegin when the multi-mode mobile station is engaged in a call with anendpoint via a conference bridge in a media server, as indicated byblock 300. Thus, the call includes an endpoint call leg that extendsbetween the endpoint and the conference bridge and a multi-mode mobilestation call leg that extends between the multi-mode mobile station andthe conference bridge. The call may have been originated by either themulti-mode mobile station or the endpoint. In addition, the multi-modemobile station may be involved in the call via either the WWAN or theWLAN. Thus, the call could have been established as set forth in FIG. 2or FIG. 4.

At some point during the call, the multi-mode mobile station's call legis unintentionally dropped, as indicated by block 302. This may occur,for example, because of a temporary degradation in signal quality or forsome other reason. The media server may detect the dropped call leg and,in response, may play an announcement to the endpoint, as indicated byblock 304. The announcement could be, for example, a pre-recorded orspeech-synthesized message asking the party at the endpoint to pleasehold because the other party has been dropped from the call and isattempting to re-connect. The announcement may be chosen so as to fillup the expected amount of time needed for the multi-mode mobile stationto re-connect. In this regard, the media server may allow the multi-modemobile station a predetermined period of time, e.g., 15 seconds, withinwhich to re-connect before releasing the endpoint's call leg.

The multi-mode mobile station also detects the dropped call leg and, inresponse, calls a predetermined re-establishment number, as indicated byblock 306. The re-establishment number could be, for example, the samedirectory number as the handoff number. The multi-mode mobile stationcould transmit the call re-establishment request via the same wirelessnetwork as the dropped call leg. Alternatively, if the other wirelessnetwork is available, then the multi-mode mobile station may registerwith the other wireless network and then transmit the callre-establishment request.

The signaling for the call to the re-establishment number is routed tothe call control system, as indicated by block 308. The call controlsystem then queries the CBI database to identify the multi-mode mobilestation's conference bridge, as indicated by block 310. For example,call control system 74 may send CBI database 72 a query that includes anidentifier of the multi-mode mobile station (e.g., an MDN, MSID, or ESN)that the multi-mode mobile station included in its call re-establishmentrequest. CBI database 72 may respond with a conference bridge identifierthat identifies the multi-mode mobile station's designated conferencebridge, i.e., the conference bridge being used for the call. The callcontrol system then routes the re-establishment call to the identifiedconference bridge, as indicated by block 312. In this way, themulti-mode mobile station's call leg is re-established, as indicated byblock 314.

4. Conclusion

Exemplary embodiments of the present invention have been describedabove. Those skilled in the art will understand, however, that changesand modifications may be made to these embodiments without departingfrom the true scope and spirit of the invention, which is defined by theclaims.

1. A method for effecting a handoff of a multi-mode mobile station froma first wireless network to a second wireless network, said multi-modemobile station having a first interface for wireless communication withsaid first wireless network and a second interface for wirelesscommunication with said second wireless network, said method comprising:receiving a call request that requests establishment of a call betweensaid multi-mode mobile station and an endpoint; in response to said callrequest, establishing a first call leg between a conference bridge andsaid multi-mode mobile station via said first wireless network;receiving a handoff request; in response to said handoff request,establishing a second call leg between said conference bridge and saidmulti-mode mobile station via said second wireless network, saidconference bridge bridging said first and second call legs; and droppingsaid first call leg.
 2. The method of claim 1, wherein said call requestoriginates from said multi-mode mobile station.
 3. The method of claim1, wherein said call request originates from said endpoint.
 4. Themethod of claim 1, wherein said first wireless network is a wirelesslocal area network (WLAN) and said second wireless network is a wirelesswide area network (WWAN).
 5. The method of claim 4, wherein said firstcall leg extends from said conference bridge to said multi-mode mobilestation via a packet-switched network and said second call leg extendsfrom said conference bridge to said multi-mode mobile station via saidpacket-switched network, a media gateway, and a circuit-switchednetwork.
 6. The method of claim 1, wherein said first wireless networkis a wireless local area network (WWAN) and said second wireless networkis a wireless wide area network (WLAN).
 7. The method of claim 6,wherein said first call leg extends from said conference bridge to saidmulti-mode mobile station via a packet-switched network, a mediagateway, and a circuit-switched network, and said second call legextends from said conference bridge to said multi-mode mobile stationvia a packet-switched network.
 8. The method of claim 1, furthercomprising: said multi-mode mobile station transmitting said handoffrequest via said second wireless network.
 9. The method of claim 8,wherein said handoff request is a request to originate a call to apredetermined handoff number.
 10. A method for communicating withmulti-mode mobile stations that can communicate with at least a firstwireless network and a second wireless network, said method comprising:associating each one of a plurality of said multi-mode mobile stationswith a particular one of a plurality of conference bridges; receiving acall request that requests establishment of a call between an endpointand a given one of said plurality of multi-mode mobile stations; inresponse to said call request, identifying a given conference bridge,from among said plurality of conference bridges, that is associated withsaid given multi-mode mobile station; and establishing said call throughsaid given conference bridge.
 11. The method of claim 10, whereinassociating each one of a plurality of said multi-mode mobile stationswith a particular one of a plurality of conference bridges comprises:defining a plurality of conference bridge identifiers, wherein each oneof said conference bridge identifiers corresponds to a particular one ofsaid plurality of conference bridges; assigning to each one saidplurality of multi-mode mobile stations a unique conference bridgeidentifier from among said plurality of conference bridge identifiers;and storing for each one of said plurality of multi-mode mobile stationsan association between a mobile station identifier of said multi-modemobile station and its conference bridge identifier.
 12. The method ofclaim 11, wherein said call request identifies said given multi-modemobile station by a given mobile station identifier.
 13. The method ofclaim 12, wherein identifying a given conference bridge, from among saidplurality of conference bridges, that is associated with said givenmulti-mode mobile station comprises: determining which conference bridgeidentifier, from among said plurality of conference bridge identifiers,is associated with said given mobile station identifier of said givenmulti-mode mobile station, to obtain a given conference bridgeidentifier; and determining which conference bridge, from among saidplurality of conference bridges, corresponds to said given conferencebridge identifier.
 14. The method of claim 13, wherein establishing saidcall through said given conference bridge comprises: establishing afirst call leg between said given conference bridge and said givenmulti-mode mobile station via said first wireless network.
 15. Themethod of claim 14, further comprising: receiving a handoff request thatincludes said given mobile station identifier of said given multi-modemobile station.
 16. The method of claim 15, further comprising, inresponse to said handoff request: identifying said given conferencebridge identifier from said given mobile station identifier; identifyingsaid given conference bridge from said given conference bridgeidentifier; and establishing a second call leg between said givenconference bridge and said given multi-mode mobile station via saidsecond wireless network, said given conference bridge bridging saidfirst and second call legs.
 17. The method of claim 14, furthercomprising: said given multi-mode mobile station detecting that saidfirst call leg has been dropped and responsively transmitting a callre-establishment request that includes said given mobile stationidentifier of said given multi-mode mobile station; identifying saidgiven conference bridge identifier from said given mobile stationidentifier; identifying said given conference bridge from said givenconference bridge identifier; and re-establishing said first call legbetween said given conference bridge and said given multi-mode mobilestation via either of said first and second wireless networks.
 18. Asystem for providing wireless telecommunications for a plurality ofmulti-mode mobile stations, comprising: a first wireless network; asecond wireless network; a plurality of conference bridgescommunicatively coupled to said first and second wireless networks; adatabase that associates each one of said plurality of multi-mode mobilestations with a particular one of said plurality of conference bridges;and a call control system communicatively coupled to said database andto said plurality of conference bridges, wherein said call controlsystem selectively routes calls to and from said multi-mode mobilestations through their associated conference bridges.
 19. The system ofclaim 18, wherein said plurality of conference bridges is provided by atleast one media server.
 20. The system of claim 19, wherein said atleast one media server is communicatively coupled to said first wirelessnetwork via a packet-switched network and is communicatively coupled tosaid second wireless network via said packet-switched network, a mediagateway, and a circuit-switched network.